Chapter Six: Photosynthesis

1. Chapter Six: Photosynthesis 6-1 Capturing the Energy in Light 6-2 The Calvin Cycle

3. 6-1 Capturing The Energy in Light I. Energy for Life Processes • Solar NRG drives reactions of LIFE (no SUN, no NRG, no LIFE). (1) A Biochemical Pathway (e.g., photosynthesis—LIGHT and DARK) • PRODUCT of 1 reaction is USED in next reaction.

4. Critical Thinking (1)A famous scientist once said that wherever in the universe life exists, some of those life-forms MUST be colored. Why would the scientist make such a statement?

6. PHOTOSYNTHESIS (2 STAGES, NOTE ON SIDE) (1) LIGHT RXNS: NRG captured from SUN; WATER split into (H+), (e-), and (O2). (2) Light NRG into Chemical NRG stored in ATP and NADPH. (3) CALVIN CYCLE: ATP and NADPH drive production of carbs using CO2.

8. II. Light Absorption in Chloroplasts (i.e., LIGHT rxns) • Begins inside THYLAKOID, stacked to form GRANUM within chloroplast.

9. (2) Granum (10-12 per chloroplast) • A stack of layered THYLAKOIDS. (3) Stroma (gelatinous matrix) • Solution SURROUNDING the THYLAKOIDS.

11. (A) Light and Pigments • Qu: HOW does a chloroplast absorb LIGHT?

13. (1) Visible Spectrum (ROYGBIV) • WHITE light can be BROKEN down into a RANGE of wavelengths (colors). (2) Wavelength (distance between crests) • Different colors = different wavelengths. (3) Pigments (clustered into PHOTOSYSTEMS) • Absorb CERTAIN colors (wavelengths) more strongly than others.

15. (B) Chloroplast Pigments (different TYPES of pigments in plant cells) • Absorb ORANGE-RED and BLUE-VIOLET colors BEST. (1) Chlorophylls (“a” & “b”) • Pigments clumped into TWO PHOTOSYTEMS (PS I and PS II).

16. (2) Accessory Pigments (NOT directly involved in LIGHT rxns) • HELP by capturing OTHER wavelengths-colors that chlorophylls cannot.

17. (3) Carotenoids (yellow, orange, and brown pigments) • Absorb wavelengths of BLUE and GREEN light, NOT absorbed by chlorophyll.

18. III. Electron Transport • Pigments grouped into CLUSTERS in THYLAKOID MEMBRANE. (NOTE: Each cluster of pigments is grouped into PS I or PS II)

20. LIGHT reactions BEGIN when accessory pigments in BOTH photosystems absorb sunlight, acquiring some ENERGY. • In each photosystem, acquired energy is PASSED quickly to other pigments until finally a specific pair of chlorophyll a molecules are reached.

21. The BIOCHEMICAL pathway following this point to making NADPH can be summarized in FIVE steps.

22. (1) Step One… • Sunlight energy forces e- to become energized in the chlorophyll a molecules of Photosystem II.

23. (2) Step Two… • Excited e- leave chlorophyll a (oxidation), so reduction must follow  e- are next accepted by a PEA in thylakoid membrane (Primary e- Acceptor).

24. (3) Step Three… • PEA donates e- to the ETC located in thylakoid membrane (ETC = Electron Transport Chain). • As the e- pass THROUGH the transport chain, they RELEASE energy, of which gets USED to pump protons (H+) INSIDE the THYLAKOID.

25. (4) Step Four… • At same time light is absorbed by PS II, light is also absorbed by PS I. Electrons move from chlorophyll a in photosystem I to another PEA. • The e- that are LOST by these chlorophyll a are REPLACED by e- that have passed through the ETC from PS II.

26. (5) Step Five… • PEA of PS I donates e- to A 2ND ETC. This chain brings e- to the side of thylakoid membrane that faces the STROMA. • There the e- combine with a proton (H+) and NADP+ (NOTE: NADP+ accepts e-  this RXN causes NADP+ to become NADPH)

27. (A) Restoring Photosystem II • In Step 4, e- from chlorophyll in PS II replaced the e- that LEAVES chlorophyll in PS I. • If the e- from photosystem II were NOT replaced, both e- transport chains would STOP, preventing photosynthesis (stopping NRG flow). • Replacement e- are provided by H2O molecules.

28. Enzyme inside thylakoid SPLITS the water into H+, e-, and oxygen as follows: 2H2O  4H+ + 4e- + O2 • Splitting of water releases e- which replaces the e- that leaves PS II when it is illuminated. • Protons (H+) made are left INSIDE the thylakoid, while oxygen diffuses OUT of the chloroplast, LEAVING the plant.

29. Critical Thinking (2)Explain how the LIGHT reactions would be affected if there were NO concentration gradient of protons across the thylakoid membrane?

32. IV. Chemiosmosis • H+ is PUMPED OUT to STROMA  releases NRG!,  used to make ATP. • NOTE: Chemiosmosis depends upon a CONCENTRATION GRADIENT of PROTONS (H+) across THYLAKOID MEMBRANE.

33. Some H+ are made as water is SPLIT inside thylakoid; other H+ are PUMPED INSIDE from outside stroma. • The NRG required to PUMP these protons is supplied by the EXCITED e- as they pass along the ETC. • Together, BOTH of these mechanisms work to build up a HIGHER [H+] INSIDE the thylakoid, and LOWER [H+] in the OUTSIDE STROMA.

34. (1) ATP Synthase (MAKES THE ATP) • [H+] gradient = NRG used by ATP Synthase in membrane to make ATP for the plant cell.

35. IMPORTANT Note: ATP Synthase is a MULTIFUNCTIONAL protein: (1) Functions as a carrier protein (moving H+) (2) Functions as an enzyme (synthesizes ATP from ADP + P)

36. (2) Adenosine Diphosphate (ADP) • Added to PO4 to make ATP by ATP Synthase. • (NOTE: NRG that DRIVES this reaction is provided by MOVEMENT of H+ from the INSIDE of thylakoid to the OUTSIDE stroma. • Together, ATP and NADPH both provide NRG for the SECOND set of RXNS in photosynthesis, known as the CALVIN CYCLE.

48. 6-2 The Calvin Cycle I. Carbon Fixation by the Calvin Cycle • Makes compounds using CO2 and NRG stored in ATP and NADPH.

50. (1) Carbon Fixation (CO2 used to make carbs, lipids, & amino acids) • Carbon atoms from CO2 are BONDED (fixed) into useful compounds. NOTE: 3 STEPS occur within STROMA during this cycle.